Servo 360 injecting current into its input pin

Phil Pilgrim (PhiPi)Phil Pilgrim (PhiPi) Posts: 22,579
edited 2020-02-16 - 02:50:47 in Robotics
I discovered this while experimenting with the Servo 360 under Blockly, but I see the same behavior under Spin. When I'm pulsing the servo while it's unpowered, I get this waveform:

servo_power_off.png

But when I power the servo, I get this:

fc499e7334d5f36329a9f9cc45822e.png

IOW, powering the servo imposes a +1V offset onto its input line (the Prop's output pin) when the pin is driven low. Because the Propeller is driving the line low, the servo must be injecting quite a lot of current into the pin at this point. It doesn't seem to affect its behavior, but I wonder about the stress on the Propeller's output port when this is going on.

I've checked this on more than one servo and see the same phenomenon. I've also checked with 5V and 7.5V servo supplies. No difference.

-Phil
480 x 360 - 11K
480 x 360 - 11K

Comments

  • Maybe a pull down resistor on the signal line would shield the prop.

    There was someone posting about their servos twitching on power-up. Mine do, too, but they are not 360s. Maybe this is still related.
  • Maybe a pull down resistor on the signal line would shield the prop.
    The pin is already being pulled down by the Prop's output being low. Yet, there's still enough current being inserted into it to raise it by a volt. A pull-down resistor would not make a dent in the offset.

    -Phil
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